Retrospective Review of Surgical Complications in Patients at High risk for Obstructive Sleep
Apnea
ABSTRACT
Background: Obstructive Sleep Apnea (OSA) is a prevalent condition in surgical population but a
vast majority of cases are undiagnosed. We compared the surgical outcomes in patients at high
risk for OSA with matched controls undergoing similar surgery.
Methods: This was a retrospective, case-control study conducted on all surgical patients
undergoing general anesthesia at our institution. On the basis of a preoperative questionnaire
or sleep study, patients were classified as high risk Obstructive sleep apnea (HR) group. This
group was matched with the same number of patients in control group (LR) using propensity
scoring and logistic regression. The two groups were compared for premorbid conditions, intraoperative course and postoperative events using cross tabulation and paired t-test.
Development of a defined complication in the postoperative period was considered as the
primary end point. Binary logistic regression was used to identify OSA as a risk factor for comorbid conditions and hazard ratios were calculated.
Results: We reviewed a total of 3593 patients and 306 patients were found to be at high risk for
OSA. HR-OSA group was found to have a higher incidence of hypertension and diabetes
preoperatively compared to controls. In the postoperative period, HR-OSA group had higher
incidence of hypoxia, reintubation and postoperative use of CPAP. The OSA group had longer
PACU stay and overall hospital stay. They were also found to have higher overall postoperative
complications.
Conclusion: Patients at high risk for OSA have a higher incidence of adverse events in the
postoperative period compared to their matched controls.
Abbreviations: OSA= Obstructive Sleep Apnea; ASA=American Society of Anesthesiologists;
CPAP= Continuous Positive Airway Pressure; PACU=Post Anesthesia Care Unit; HR=High risk
group; LR=Low Risk group.
Word Count 249
INTRODUCTION:
Background: Obstructive sleep apnea (OSA) is a clinical syndrome characterized by repeated
occlusions of upper airway during sleep resulting in sleep fragmentation and nocturnal
hypoxemia. Symptomatic OSA affects approximately 2% and 4% of women and men
respectively, but the overall prevalence of sleep disorder was estimated as 9% for women and
24% for men between the ages of 30 and 60. 1 Per a survey study in elective surgical patients
using Berlin questionnaire, 24% patients were found to be at risk of OSA.2
Of concern is the fact that the majority of surgical patients with OSA are undiagnosed. There
are various screening tools for the diagnosis of OSA in surgical patients, but their validity for use
in perioperative setting is still under question. The Berlin questionnaire is the most widely used
tool to detect OSA.3-5 The American Society of Anesthesiologists (ASA) checklist is a consensus
of the task force to identify high-risk patients. 6 This checklist consists of 12 items that include
predisposing physical characteristics, history of apparent airway obstruction during sleep and
daytime somnolence. Snoring, tiredness, observed stopped breathing, blood pressure (STOP
questionnaire) is also a concise and easy to use tool to identify patients at risk for having OSA.7
An alternative scoring model combining STOP questionnaire and BANG (BMI, age, neck
circumference and sex/gender) further improves the sensitivity.8 Polysomnography however
remains the gold standard for diagnosis of OSA, but it can be inconvenient and time consuming
in the perioperative setting.
Various studies have been done for the validity of these questionnaires for use in clinical
practice. A study by Chung et.al concluded that Berlin questionnaire and ASA check list have a
high degree of sensitivity and negative predictive value.4,9 STOP questionnaire has been
validated for use in perioperative setting.7
OSA is associated with hypertension, cardiac dysrhythmias, myocardial infarction, heart failure
and stroke. 10-19 Due to the associated airway pathology and depressive effects of anesthetic
agents on respiration and pharyngeal muscle tone, patients with OSA are at a higher risk for
respiratory complications. Additionally, airway management of these patients has been shown
to be challenging for the anesthesiologists in view of difficulty in securing the airway.12,20,21
Patients at high risk for OSA may not actually carry a diagnosis of OSA due to lack of a
confirming sleep study and their subtle symptoms which the patients may not even recognize.
Objectives: The primary objective was to compare the surgical outcomes in patients at high risk
for OSA with control patients undergoing similar surgeries.
METHODS:
Study design: We conducted a retrospective case control study on patients who have
undergone surgery during the period from September 2007 to September 2009.
Setting: This study was reviewed and approved by the Institutional Review Board Committee at
the VA Western New York Healthcare System (VAWNYHCS) Buffalo, NY. The approval number
was 2010-00704. At our institution, we have incorporated a preoperative protocol for
annotating OSA history with the aid of a questionnaire based on ASA checklist6 in our
preoperative evaluation for all patients (Table 5). This preoperative evaluation is done in a
preoperative clinic by anesthesia nurse practionners routinely. In case of emergent surgeries,
the evaluation is done by the anesthesia residents or anesthesiologists.
All patients between the ages of 18-80 years who had general anesthesia were included.
Patients with regional anesthesia or more than one surgery during this period were excluded.
Patients with OSA risk score equal or greater than 5 were identified as high OSA risk (HR) and
the remaining patients with a score less than 5 were categorized as low OSA risk (LR). Also the
patients who were already diagnosed with OSA on the basis of sleep study were included in the
HR group.
Clinical data from 3,593 patients were collected through a detailed review of the preexisting
preoperative and clinical records by members of the research team (anesthesia resident, a
research associate and a trained anesthesiologist) from Computerized Patient Record System
(CPRS) database in use at the VAWNYHCS.
A detailed review of the medical records, the preoperative assessment as performed earlier,
was done to extract data about patient demographics, American Society of Anesthesiologists
(ASA) physical status, airway assessment on the basis of Mallampati classification and existing
co-morbidities. From the preoperative data, patients at high risk OSA and low risk OSA were
also identified as mentioned earlier. Intraoperative data included type, invasiveness and
duration of the surgical procedure, type of anesthesia, type of airway method used and amount
of opioids administered. Postoperative complications of interest included hypoxia, defined as
one or more incidences of pulse oximetry value ≤ 90% on 2-3L/min by nasal cannula;
reintubation; development of new onset atrial fibrillation for at least 3 minutes; cardiac
ischemia defined as ST-T wave changes in the postoperative period; hemodynamic instability
defined as SBP<90 or DBP<50 or a change >20% from baseline; myocardial infarction with
evident elevation of biomarkers or evidence of CVA on the basis of a new onset of neurological
deficit. Duration of post anesthesia care unit (PACU) and hospital stay or readmission to the
hospital within 24 hours of discharge was also evaluated. Occurrence of single complication in
postoperative period was compiled into a single composite event for further statistical analysis.
All continuous variables were expressed as mean± SD and categorical variables were expressed
as percentage of patients in the data. Type of surgery was included in the questionnaire for
calculating OSA risk and hence was already considered for classification of patients. Since there
were significantly more ASA physical status (PS) 3 and 4 among the high OSA risk group
compared to the low risk group which had more ASA PS-2 and 3 (p value 0.002), ASA PS was
included in the matching variables.
We compared the PACU stay as a marker for postoperative morbidity in all patients. The
patients who recovered from anesthesia in the PACU were deemed ready for discharge from
the PACU only after a staff anesthesiologist signed them off when they met modified Aldrete
scoring22 criteria. The discharge time was considered the time that they left PACU or when
considered ready to be discharged. The patients who were transferred to the ICU were
excluded from the PACU stay as these patients completed their recovery from anesthesia in the
ICU and therefore never went to PACU.
Data Measurement and Statistical analysis: The incidence of complications was compared
between the two groups and a primary outcome variable defined as the occurrence of single
postoperative complication or in hospital death. The preoperative comorbidities, intraoperative
and postoperative complications data were entered into a Microsoft excel spreadsheet 2007
Inc. NCSS version 2007 (NCSS, LLC. Kaysville, UT) software was used for data analysis. Data were
matched for relevant preoperative clinical co-morbidities and all factors that had P<0.1 on the
basis of univariate analysis, which implied that a clinical trend may be possible, were included in
calculating the propensity score. HR patients were matched 1:1 with LR patients on the basis of
Age, ASA physical status and airway surgery using propensity scoring and multivariate logistic
regression. Statistical Analyses were performed by using chi square test for categorical variables
and two-sample paired t-test for continuous variables. Statistical analyses were performed on
both prior to and after matching. The default for binary data was taken as zero; and for
numerical variables they were replaced by the median value. Binary logistic regression was
used to find out the predictive value of OSA in certain comorbid conditions. Hazard ratio was
calculated for all co-morbidities. A P value <0.05 was considered statistically significant.
RESULTS:
Out of 3,593 patients studied, on the basis of patient characteristics, 306 patients were found
to have high OSA risk (HR) and compared to 3286 patients in the low risk group (LR). There was
no significant difference between the low risk and the high risk groups on the basis of gender,
age or race. There were more ASA PS-3 and 4 patients in the high risk group than the low risk
group. The incidence of premorbid conditions especially coronary artery disease, hypertension,
diabetes and chronic obstructive pulmonary disease was higher in the HR (Table 2). In addition,
the HR patients have a higher association with coronary artery disease, diabetes mellitus and
hypertension compared to the LR group as evident from the hazard ratio (Fig 1).
There were more patients with airway Mallampati class 3 and 4 in the HR than the LR group (p
value<0.01). Intraoperative airway management was more likely to be attained by tracheal
intubation in the HR than the LR group (73.4% vs. 62.2%) (Table3). The patients in the HR group
were found to have significantly higher incidence of postoperative hypoxia, and reintubation.
More patients from the HR group required CPAP in the postoperative period and also the more
patients were admitted as inpatients after surgery (Table 4)
The HR group patients were matched 1:1 with 306 LR controls and statistical analyses were
performed. The patients in the HR were similar to the matched controls with respect to sex,
race, ASA physical status and presence of airway surgeries. Patients with HR had higher
incidence of hypertension (80.4% vs. 68.6%, P<0.001) and diabetes mellitus (44.1% vs. 27.1%,
P<0.0001). However, there was no significant increase in the incidence of coronary artery
disease (CAD) or chronic obstructive pulmonary disease (COPD) among the HR patients.
In the postoperative period, the patients in the HR group had a higher incidence of
postoperative hypoxia as defined earlier, compared to the matched LR controls (16.8% vs.
10.2%, p< 0.01). Fifteen patients from the HR group while only two patients from the LR group
were reintubated in the postoperative period (4.9% vs. 0.7%, p<0.001) and transferred to the
ICU (Table 4). More patients in the HR group needed postoperative use of continuous positive
airway pressure (CPAP) (7.2% vs. 1.3%, P= 0.0003). This figure included outpatients with
previous CPAP usage who continued their therapy during postoperative period. Overall, the
incidence of an aggregate adverse event was 25.4% in the HR group while its occurrence was
only 17.4% in the LR matched controls (p<0.01).
Patient in the HR group stayed in the PACU longer (113±54 minutes) when compared to their
matched group with low OSA risk (99±52 minutes, P<0.01). The likelihood of hospital admission
after surgery was similar in both groups. Similarly, after admission to hospital, there was no
difference on the overall lengths of hospital stay (11.3±18.6 days vs. 13.7±35.8 days).
We did not find any increase in mortality in the HR group.
DISCUSSION:
Our study demonstrates that a high risk for OSA translates into a greater incidence of adverse
events in the postoperative period. Adverse events included hypoxia, the incidence of
reintubation, increased need for CPAP use and a longer stay in PACU in the HR group.
A significant complication that we found in the high risk OSA group postoperatively was the
incidence of reintubation which was significantly higher compared to the matched controls.
During the analysis of patient demographics we found higher incidence of MP airway class 3
and 4 in high OSA risk group. The existing literature does suggest that OSA patients have a
higher incidence of difficult intubation and airway problems owing to obesity and other physical
characteristics,12,20,21 but there has been no report of a higher incidence of reintubation in
postoperative period. The higher frequency of reinsertion of endotracheal tube could
contribute to high mortality and morbidity in these patients with an existing risk for difficult
airway, and has serious implications for the anesthesiologists caring for these patients.
There were two retrospective reviews on postoperative complications in diagnosed OSA
patients.23,24 In one study by Liao and colleagues on known OSA patients who underwent
elective surgery other than airway surgery, they found that patients with diagnosed OSA have
an increased incidence of oxygen desaturation.23
Gupta et al studied patients with OSA undergoing orthopedic surgeries, and found that OSA
patients had higher incidence of respiratory and cardiac complications in the postoperative
period. The length of hospital stay and unplanned ICU admissions were higher. 24 However, we
were unable to demonstrate such an association among our patients. Gupta and colleagues
demonstrated a length of stay at 6.8±2.8 days and 5.1±4.1 days for the OSA and the control
group respectively. We found no difference in length of stay between the HR and the LR group.
Most likely explanation for this discrepancy is the fact that we have generally elderly patient
population at our institution. Higher rate of non OSA related comorbidities increasing the inhospital stay in these patients may have contributed to this observation.
Both the above cited studies had respiratory complications and oxygen desaturation as the
commonest complications in the postoperative period.25-27 The study by Sabers et.al showed
that in known OSA patients undergoing outpatient surgical procedures excluding airway
surgeries, the diagnosis of OSA was not a risk factor for unplanned admissions.28 Since this
study included less invasive, ambulatory procedures, projection of the results to the general
surgical procedures cannot be done. In this study we found that HR-OSA patients were more
likely to be admitted as inpatients compared to LR group, but after matching this observation
was not significant. We acknowledge the fact that this study lacked the data on segregation of
patients with respect to their inpatient or outpatient status preoperatively which poses a
limitation to our finding.
The majority of studies have been performed on patients with a confirmed diagnosis of OSA.
However, this study included the general population who were identified at their preoperative
visit to be at higher OSA risk on the basis of the preoperative questionnaire.
Although there was an association between OSA risk scoring and positive Polysomnography in
this study as the patients who had a known diagnosis of OSA were higher in the HR group (P
value 0.000), this correlation is not 100%. This reemphasizes that inclusion of ASA practice
guidelines into the preoperative evaluation could create a high index of suspicion for OSA. This
simple screening questionnaire reasonably correlates with the sleep study proven OSA.
We also observed that higher number of patients in the HR group received CPAP in the
postoperative period. As mentioned before, there were more patients in the HR-OSA group
who were found to have diagnosed OSA who are supposed to be using CPAP as treatment, as a
direct consequence, the number of patients using CPAP were higher in the HR-OSA group,
which is not a surprising finding. Overall, the evidence regarding the benefits of postoperative
CPAP is relatively poor as it has been overlooked in most of the existing studies on OSA, and if
found, the sample size is so small that it is not feasible to draw conclusions.
Limitations: This is a retrospective case control study, hence there remain many limitations of
the retrospective chart review like incomplete or missing documentation, poorly recorded
information in certain cases and also difficult to ascertain cause and effect relationship.
Another important limitation was predominance of male gender among veterans. Preexisting
literature suggests that OSA is a more prevalent condition in males compared to the
females.2,29,30 This gender distribution contributes to higher number of OSA patients in our
study.
We found that hypertension, diabetes mellitus and coronary artery diseases were significantly
higher in the OSA population and overall the ASA physical status was higher in OSA population
due to the presence of these co-morbidities, therefore we could not include these in our
matching criteria. We also could not match for BMI as it was obviously higher in the patients
with high risk for OSA. . This is consistent with the literature as OSA has been linked to various
co-morbidities10,13-19 and BMI tends to be higher in the OSA risk patients.20,21
Our results suggest that patients with OSA have higher incidence of morbidity or perioperative
adverse events, which implies that we need to devise specific management strategies for OSA
patients in the perioperative period to reduce the economic burden on the health care system.
It is not feasible in the perioperative period to get sleep study done on every patient due to the
time constraints and inconvenience to the patients. In this setting, screening the patients at the
time of preoperative visit, using simple yes/no questionnaires as suggested by the ASA practice
guidelines and other questionnaires like Berlin and STOP or STOP-BANG can help identify the
high risk patients and specific measures can be taken beforehand to minimize the adverse
perioperative outcomes.
All these results are derived from retrospective data analysis. More prospective studies in
future on the perioperative complications in the high risk and known OSA patients are needed
in future to support these findings. Also, the efficacy of CPAP has not been established in the
perioperative setting. We need more prospective studies on the benefits of CPAP in the
postoperative period.
Table 1: Patient Demographics
LR
HR
N=3286
N=307
P value
N=306
63.2±14.7
61.5±12.8
0.02
61.94±12.9
61.5±12.8
0.82
143(4.4)
12(3.9)
0.89
12(3.9)
12(3.9)
1.000
3142(95.6)
295(96.1)
294(96.1)
294(96.1)
Black
418(12.7)
36(11.7)
45(14.8)
36(11.8)
White
2830(86.1)
267(87.0)
256(83.9)
265(86.9)
38(1.2)
4(1.3)
4(1.3)
4(1.3)
PS-1
70(2.1)
3(1.0)
2(0.7)
2(0.7)
PS-2
1033(31.4)
68(22.1)
62(20.3)
68(22.2)
PS-3
1669(50.8)
171(55.7)
167(54.6)
171(55.9)
PS-4
504(15.3)
63(20.5)
75(24.5)
63(20.6)
PS-5
10(0.3)
2(0.7)
0(0.0)
2(0.7)
1
1952(59.4)
140(45.6)
167(57.4)
133(44.5)
2
225(6.8)
8(2.6)
27(9.3)
8(2.7)
3
960(29.2)
133(43.3)
80(27.5)
132(44.1)
4
142(4.3)
26(8.5)
17(5.8)
26(8.7)
Age
LR
HR N=306
P value
Gender
Female
Male
Race
Others*
0.86
0.670
ASA
0.0020
0.498
Airway§
0.00000
Data presented as mean ± SD or No. (%), as appropriate
*Others: Asians, Pacific islanders, Native Indians
§Airway: Mallampati classification of airway into 4 classes
Abbreviations: OSAR: Obstructive sleep apnea risk; ASA PS: American Society of
Anesthesiologists Physical status.
0.00001
Table 2: Preoperative comorbidities:
LR
HR
LR
HR N=306
N=3286
N=307
P value
N=306
CAD
837(25.5)
101(32.9)
0.004
83(27.1)
101(33.1)
0.106
CKD
299(9.1)
37(12.1)
0.08
23(7.5)
37(12.1)
0.057
A Fib
253(7.7)
24(7.8)
0.94
19(6.2)
24(7.8)
0.439
HTN
2095(63.8)
247(80.5)
0.00000
210(68.6)
246(80.4)
0.00084
Diabetes
862(26.2)
136(44.3)
0.00000
83(27.1)
135(44.1)
0.00001
COPD
751(22.9)
87(28.3)
0.029
78(25.5)
87(28.5)
0.398
P value
Data are presented as No. (%)
Abbreviations: CAD: Coronary artery disease; CKD: Chronic kidney disease; A Fib: Atrial
Fibrillation; HTN: Hypertension; COPD: Chronic airway obstructive disease
Table 3: Airway Method
LR
HR
N=3286
N=307
161(4.9)
15(4.9)
ET Tube
2023(62.2)
224(73.4)
LMA
1071(32.9)
66(21.6)
Facemask
LR
P value
<0.001*
HR N=306
N=306
P value
20(6.6)
15(4.9)
202(66.4)
223(73.4)
82(27.0)
66(21.7)
0.17
Data are presented as No. (%)
Abbreviations: ET: Endotracheal; LMA: Laryngeal Mask Airway
*In the unmatched data, more patients received tracheal intubation, this could be due to the
assumption that anesthesiologists have a tendency for intubation in patients with a anticipated
difficult airway.
Table 4: Postoperative complications:
LR
HR
N=3286
N=307
P value
N=306
22(0.7)
23(7.5)
<0.001*
4(1.3)
22(7.2)
<0.001*
334(10.2)
51(16.6)
<0.001*
31(10.2)
51(16.8)
<0.01*
Reintubation
31(0.9)
15(4.9)
<0.001*
2(0.7)
15(4.9)
<0.001*
MI
15(0.5)
0(0.0)
0.24
3(1.0)
0(0.0)
0.08
Ischemia
13(0.4)
3(1.0)
0.14
2(0.7)
3(1.0)
0.65
A Fibrillation
67(2.0)
9(2.9)
0.30
5(1.6)
9(3.0)
0.28
CVA
6(0.2)
0(0.0)
0.45
0(0.0)
0(0.0)
NC
Inpatients
1955(59.8)
209(68.5)
<0.001*
201(65.9)
208(68.4)
0.51
Outpatients
1314(40.2)
96(31.5)
104(34.1)
96(31.6)
Readmission
25(0.8)
0(0.0)
0.29
0(0.0)
0(0.0)
NC
ICU admission
709(21.6)
87(28.3)
<0.01*
85(27.8)
87(28.4)
0.89
Death
471(14.3)
49(16.0)
0.43
56(18.3)
49(16)
0.45
CMPEVENT
511(15.6)
77(25.1)
<0.001*
53(17.4)
77(25.4)
0.01*
CPAP use
Hypoxia
LR
HR N=306
P value
Data presented as mean ± SD or No. (%), as appropriate
Abbreviations: CPAP: Continuous positive airway pressure; MI: Myocardial Infarction; A fib:
Atrial Fibrillation; CVA: Cerebrovascular accident; ICU: Intensive care unit; CMPEVENT:
Composite event; NC: Not calculated
Table 5: Identification and Assessment of OSA:
(Preoperative questionnaire as used at VAWNYHCS)
Obstructive Sleep Apnea Risk Assessment:
The provider will complete the obstructive sleep apnea RISK assessment and evaluation
of home use of CPAP/NIPPV compliance.
1. Identification and Assessment of Obstructive Sleep Apnea:
a. If a sleep study (Polysomnography) has been performed, the results should be
used to determine the perioperative management of the patient.
b. Results from a previous sleep study will be utilized unless it was conducted
longer than five years ago, before significant weight gain or another current
potentially associated factor. In these cases, another sleep study will be
conducted.
c. In the absence of a formal sleep study, if a patient has signs or symptoms in
two or more of the below categories there is a significant probability that the
patient has obstructive sleep apnea.
2. Clinical signs and symptoms suggesting the possibility of obstructive sleep apnea:
a. Predisposing Physical characteristics
 BMI 35kg/m2
 Neck Circumference 17 inches (men) or 16 inches
(woman)
 Craniofacial abnormalities affecting the airway
 Anatomical nasal obstruction
 Tonsils nearly touching or touching in the midline
b. History of apparent airway obstruction during sleep (two or more
of the following are present: if patient lives alone or sleep is not
observed by another person, then only one of the following needs
to be present).








Snoring (loud enough to be heard through a closed
door)
Frequent snoring
Observed pauses in breathing during sleep
Awakens from sleep with choking sensation
Frequent arousals from sleep
Somnolence (one or more of the following is present)
Frequent somnolence or fatigue despite adequate
“sleep”
Falls asleep easily in a non-stimulating environment
(eg watching TV, reading, riding in or driving in a car
despite adequate “sleep”)
3. Obstructive Sleep Apnea Scoring System
a. If the results of a sleep study are available utilize the sleep lab’s classification as mild,
moderate or severe or the Apnea-hypopnea index (AHI) number.
Severity of Obstructive
Sleep Apnea
Adult AHI
Points
None
0-5
0
Mild
6-20
1
Moderate
21-40
2
Severe
>40
3
(Subtract 1 point for pre and post op CPAP/ NIPPV. Add one point if resting PaCO2 > 50 mm Hg)
Surgery
Anesthesia
Superficial
Local/ block without
sedation
Points
0
Superficial
Moderate Sedation or GA
Neuraxial with minimal
sedation
1
Peripheral Surgery
General Anesthesia
2
Major Surgery
General Anesthesia
3
Airway Surgery
Post op opioid requirement
None
Low dose oral
Points
0
1
High dose oral, parenteral
2
Neuraxial opioids
3
Overall score = patient severity score + higher of surgical or postoperative Opioid score.
Adapted from ASA practice guidelines for perioperative management of patients with OSA6
Figure Legend:
Figure 1: Binary Logistic regression analysis for predictive value of OSA for different comorbidities with Odds ratios and 95% confidence interval of OSA patients vs. those with no OSA
risk.
Abbreviations: COPD= Chronic Obstructive Pulmonary Disease
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